The yeast genome contains many transposable elements which can transpose from one site in the genome to a new site and which can insert into or near structural genes leading to mutant phenotypes. The expession, reversion and further mutation of these mutant genes is controlled by genetically unlinked elements known as Suppressor-mutators (Spm). These Spm regulatory elements will be isolated from total yeast DNA and analyzed by Southern hybridization to determine whether they represent unique or repeated yeast DNA sequences and whether they share homology with the elements whose behavior they control. We will isolate and characterize mutations in the transposable elements, in their regulatory elements and in other yeast genes involved in the interactions between these elements. We will examine the effect of mutations in the Spm elements on the transcription of transposable elements and on the frequency of transposon-mediated DNA rearrangements. In addition, we will establish an assay which measures the excision of a transposable element from its flanking DNA in vitro in cell-free yeast extracts. The long term goal of this research is an understanding of the molecular mechanisms of transposon-mediated recombination events and of the interactions between the transposable elements and their regulators. Yeast DNA sequences which act as localized stimulators of homologous recombination events will be isolated from total yeast DNA. The structure of these hotspots will be determined and the recombination events promoted by these sites will be characterized.